1. Field of the Invention
The present invention relates to a semiconductor optical device which can be used as a pick-up for an optical disk and a method for fabricating the optical device.
2. Description of the Prior Art
A semiconductor laser diode of nitride (GaN or InxGa1-xN(0 less than xxe2x89xa61)), recently proposed, has a thin film structure comprising an n-type GaN layer, n-type cladding layer, active layer of a multilayered structure (a structure of single or plural quantum wells), p-type cladding layer and p-type GaN layer which are sequentially grown on a substrate of sapphire. After the laser diode film is grown, a dry etching process is performed to etch regions of a surface of the grown laser diode film in which n-type and p-type electrodes, necessary to the operation of the laser diode, are to be formed. Then, the electrodes are formed in the etched surface regions, thereby completing the semiconductor laser diode structure.
At this time, the dry etching process or a cleaving process is finally used for the formation of an optical cavity mirror which is an important factor for laser oscillation.
In a conventional process of fabricating a GaAs-based semiconductor laser diode, as shown in FIG. 1, a thin film of the same material as that of a substrate is grown on the substrate, thereby making it easy to form an optical cavity mirror using a cleaved region in a crystal structure. However, in a nitride semiconductor laser diode where a thin film of nitride is grown on a (0001) sapphire substrate, as shown in FIG. 2, the sapphire substrate and nitride film are different in lattice constant and, further, crystal orientations thereof are swerved 30 degrees on the basis of a c-axis from each other. These make it very difficult to form an optical cavity mirror using a common, cleaved region of the sapphire substrate and nitride film. For this reason, it is known that the dry etching process is mainly used for the formation of an optical cavity mirror.
On the other hand, a normal metal organic chemical vapor deposition (MOCVD) method may be used for the growth of a nitride semiconductor film. In this case, the grown nitride film has a threading dislocation density in a range of 108xcx9c109cmxe2x88x922, which is much higher than that of a conventional GaAs film.
In the case where a silicon oxide pattern-based epitaxial lateral overgrowth (ELOG) method is used to grow a nitride film, it is known that the grown nitride film has a threading dislocation density of near zero. In practice, it has been reported that the operation life time of a laser diode which is fabricated from a semiconductor film structure grown by ELOG method is lengthened from the existing several hundred hours up to several thousands to a myriad of hours.
However, in the fabricated semiconductor laser diode of the above report, an optical cavity mirror is formed in a dry etching manner, which must be followed by complex processes such as a photolithography after the formation of electrodes. The operation of the laser diode may be damaged due to contamination in those processes or a physical and chemical shock to the mirror formed during the dry etching. Such damage is not negligible.
In a ridged semiconductor laser diode employing a ridged structure for the focusing of current, the dry etching process is required once more for the formation of a ridge.
As a result, the development of a new process is required which is capable of excluding a dry etching process being liable to give a physical and chemical shock to an optical cavity mirror in a fabricating process of a GaN semiconductor laser diode and simplifying complex processes following the formation of the mirror, thereby increasing the reliability of the semiconductor laser device.
A conventional GaN semiconductor laser diode and a method for fabricating the same have the following disadvantages.
A dry etching process and the associated processes are performed to form an optical cavity mirror and a ridge, thereby making the entire processing very complex.
Further, the above-mentioned processes are liable to give a physical and chemical shock to a surface of the optical cavity mirror, resulting in a degradation in the reliability of the device.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a semiconductor optical device and a method for fabricating the same, in which a dry etching process conventionally used for the formation of an optical cavity mirror and a ridge is omitted, thereby simplifying a fabricating process of the device and increasing the reliability of the device.
In accordance with one aspect of the present invention, there is provided a semiconductor optical device comprising a substrate; a semiconductor electrode layer of a first conductive type formed on the substrate and having a groove formed to a desired depth therein; a semiconductor layer of the first conductive type formed from side walls of the groove up to a part of the semiconductor electrode layer of the first conductive type on the periphery of the groove; a cladding layer of the first conductive type, an active layer of the first conductive type, a cladding layer of a second conductive type and a semiconductor electrode layer of the second conductive type sequentially formed on the semiconductor layer of the first conductive type; and electrodes of the first and second conductive types formed respectively on the semiconductor electrode layers of the first and second conductive types.
In accordance with another aspect of the present invention, there is provided a method for fabricating a semiconductor optical device, comprising the steps of sequentially forming a semiconductor electrode layer of a first conductive type and a first silicon oxide film on a substrate; patterning a desired region of the first silicon oxide film into a desired shape to partially expose the semiconductor electrode layer of the first conductive type and etching the resultant exposed region of the semiconductor electrode layer of the first conductive type to a desired depth to form a groove; forming a second silicon oxide film on a bottom of the groove; growing a semiconductor layer of the first conductive type from side walls of the groove up to a part of the first silicon oxide film on the periphery of the groove; sequentially growing a cladding layer of the first conductive type, an active layer of the first conductive type, a cladding layer of a second conductive type and a semiconductor electrode layer of the second conductive type on the semiconductor layer of the first conductive type to form an optical cavity mirror; and forming electrodes of the first and second conductive types respectively on the semiconductor electrode layers of the first and second conductive types.